Tension is escalating around the skirmish-prone western maritime border as Pyongyang vowed retaliation against South Korea’s firing of warning shots Friday at North Korean fishing vessels that violated the territorial waters.

The communist regime Saturday accused the South of a “shooting rampage” against boats owned by another country.

“What remains to be done now is a powerful strike of our front units which know no bounds,” Pyongyang’s official news agency said.

On Friday around noon, South Korean patrol vessels spotted six North Korean crab boats south of the so-called Northern Limit Line. The navy fired two warning shots at about 3 p.m. as they resisted dropping back despite broadcast warning messages.

None of the ships were hit and they retreated to their waters by 4 p.m., the Defense Ministry said.

Later Saturday, another North Korean fishing vessel was reported to have violated the boundary.

On top of about 100 North Korean vessels, some 300 Chinese boats were reportedly operating in the waters on Friday for the peak crab season.

But an official at the Joint Chiefs of Staff dismissed Pyongyang’s accusation, saying “the fishing boats intruding on the NLL were certainly North Korean. The differences in appearance between Chinese and North Korean vessels are even clearly seen with the naked eye.”

The incident marked the South’s first resort to warning shots in two years to force North Korean fishing boats back.

It is the latest in a series of border intrusions in recent weeks. Seven vessels crossed the line twice and backed away after warning signs from South Korean high-speed boats on Sept. 12. Separate groups of two to three boats repeatedly came in sight again on Sept. 14, 15 and 20, the JCS reported.

Pyongyang does not recognize the NLL, arguing that it was drawn unilaterally by the U.S.-led United Nations forces after the 1950-53 Korean War. It demands a new boundary farther southward.

Military officials here largely interpret the incursions as the North’s attempt at swaying the December presidential election by stirring military tension.

Still, concerns are brewing that a skirmish is prone to quickly escalate into a major naval clash that entails exchanges of machine gun and artillery fire.

At the time of warning shots, the South’s military also deployed a fighter jet armed with air-to-surface and air-to-air missiles, Yonhap News cited an unnamed military source as saying.

It also found that the outer doors of some shore batteries across the border were open. The heavily militarized country keeps most their emplacements in caves and other fortified areas and thus soldiers must open their outer doors to fire off gun.

“If North Korea’s military launches provocations near the NLL, we will operate the joint forces to promptly and sternly respond,” the source said, asking not to be named because he was not authorized to released the information.

With the heightened alert status in the region, Seoul and Washington are believed to have begun intensifying cooperation in intelligence gathering to better prepare for any urgency.

The NLL is close to Yeonpyeong Island, which the North bombarded with artillery in November 2010, killing four South Koreans. Months before that, the North torpedoed a South Korean warship in the nearby waters that claimed 46 lives.

In 1999, a major battle broke out after North Korean naval vessels and crab boats violated the boundary. At least 20 North Korean sailors are believed to have had been killed and 30 injured, whereas seven South Korean sailors were hurt and two ships were damaged.

Similar bloody clashes took place in 2002 and 2009.

The two Koreas technically remain at war since the war ended with a truce rather than a peace treaty.

By Shin Hyon-hee (heeshin@heraldcorp.com)